1. Field of the Invention
The present invention relates to a howling protective apparatus for a sound device comprising a microphone, an amplifier and a speaker.
2. Prior art
With a speaker device in which a microphone and a speaker are located in the same sound field, direct sound entered in the microphone is amplified by an amplifier and then sounded into space through the speaker Indirect sound, outputted from the speaker, again enters into the microphone. At that time, if the amplification degree of the speaker device is large in excess or any resonance occurs to a particular frequency because of the effect from the transmission frequency characteristics of the sound field, the secondary input affected by the indirect sound output from the speaker becomes higher in level than the primary input entered in the microphone, and inputted into the microphone. Since these operations are repeated sequentially, the sound in the particular frequency is amplified and enhanced until the sound signal saturates in any component medium in the loop of the sound system. The phenomenon is called howling.
In order for preventing howling, various methods for suppressing the feedback quantity of speaker sound to the microphone have been proposed conventionally.
For example, (1) a microphone with a directivity to a sound source is used and located as near the sound source as possible, thereby increasing the ratio of a direct sound to an indirect sound. (2) A speaker of a sharp directivity is used to suppress positive feedback of an indirect sound. (3) Transmission frequency characteristics are limited near peak values, using an equalizer. (4) Phase is shifted. (5) Frequency fluctuation is given. (6) A switch is changed over to shut out speaker sound to enter the microphone.
Even when these methods are introduced, said means (1) and (2) are not applicable to a moving sound source. In (3), there are many howling frequencies besides near peak values. In (4), the effect of shifted phase is not even but depends on frequency, sound pressure level, reflection, etc. Methods (5) sometimes brings out howling. The application of (6) is limited. Therefore, it is difficult to construct a sound system free from howling.